The invention is not restricted to a special type of motor vehicle, although the invention is particularly applicable to heavy wheeled motor vehicles, such as trucks or lorries and buses, and the invention will for that sake hereinafter primarily be discussed for that field of use for illuminating the invention but accordingly not in any way restrict it thereto.
The fuel used in such an engine may be of any conceivable type and diesel and ethanol may be mentioned as examples. Such a compression ignited engine may offer poor possibilities to use engine braking when braking a motor vehicle. This is the reason to provide especially heavy motor vehicles with a compression release brake system for providing additional braking of the vehicle in particular when driving downhill. A compression release brake system causes braking by opening exhaust valves of the engine at positions of the operation cycle of the cylinders resulting in a modified compression in the cylinders and by that a braking action upon a crank shaft of the engine with respect to no such opening of the exhaust valves. Such exhaust valves opening may especially be carried out in connection with reaching the top dead center TDC of the piston in the cylinder after the compression cycle, so that air trapped in the cylinder is released and the piston will not receive any force from any gas pushing it down when moving away from said top dead center.
However, the function of such a compression release brake system may of course for different reasons be degraded, which may for instance be due to a malfunction related to only one or a few of the cylinders of the engine, such as an incorrectly functioning valve rocker arm for the exhaust valve of such a cylinder which may depend upon an obstruction in an hydraulic line to a chamber to be filled with hydraulic fluid for triggering modified opening of the exhaust valve or a malfunction of an hydraulic valve controlling the supply of hydraulic fluid to a said chamber. The driver of the vehicle may then bring the vehicle to a workshop and complain about the deteriorated braking effect of the compression release brake system. However, the workshop will then not have any possibility to check the function of the compression release brake system, since it is only possible to activate this system when the engine is running and the system has the required hydraulic fluid pressure making it mostly impossible to point out for which of the cylinders the valve rocker arm associated with the exhaust valve does not function correctly when activating the compression release brake system. This may then result in a replacement of said valve rocker arms for all the cylinders and a considerable unnecessary cost although perhaps only one valve rocker arm functions incorrectly when activating the compression release brake system. It is even possible that the replacement of all valve rocker arms will not result in a compression release brake system functioning perfectly would a malfunction for any of said valve rocker arms be dependent upon an obstruction of a said hydraulic fluid line.
US 2003/0115944 A1 discloses a method for checking the function of a compression release brake system, in which this system is activated and deactivated and differences in a response of the engine are compared with expected such responses. WO 2013/125439 A1 discloses another method for checking the function of a compression release brake system, in which the number of revolutions of an engine versus time is examined with the compression release brake system in an active state and in an inactive state. However, the methods disclosed in these documents have the problem just mentioned with respect to pointing out for which of the cylinders the compression release brake system does not function would it be determined that the compression release brake system as a whole has a degraded function.